The present invention relates generally to solids handling and in particular to feeder valves, having little or no air loss, used in pneumatic conveying systems.
In pneumatic conveying solid materials (powder, sand, gravel, coal, and the like as well as agricultural products such as seed and foodstuffs) must be moved from a storage facility be it a silo, a bin, a bunker, or similar, into a pneumatic conveying system. The storage facility is generally at atmospheric pressure; whereas, the conveying system is at a different pressure. Generally, the pneumatic system is at a higher pressure, but there are some vacuum pneumatic conveying systems. The conveying system pressure must be isolated from the storage hopper (bin, silo, etc.), and an airlock type valve is generally used for this purpose.
The airlock valve can take several forms. In some systems, the valve can be a simple gate or ball valve, which opens when the conveying system is de-pressured. This allows material to enter the conveying system. The valve is closed, the system pressured, and the material conveyed. This type of system is a batch process system and cannot deliver material at continuous rate.
In order to deliver material at a continuous rate, the conveying system must remain pressured at all times. Thus, in a continuous system, the airlock valve must be capable of delivering material from the hopper and into the system while maintaining system pressure. Finally, in continuous systems, the operator generally wishes to deliver a certain rate of material over time. Thus, the airlock valve should be capable of xe2x80x9cmeteringxe2x80x9d the material from the hopper.
The past art has generally employed a xe2x80x9cstar-valvexe2x80x9d that obtains its name from the valve internals, which are shaped like a star. The star is formed about a shaft and is rotated within a circular valve body. The valve internals are a series of circular open sectors starting at the shaft and extending towards the circular wall of the valve body. The width of the sectors is set by the width of the valve body. The valve is placed between the hopper and the pneumatic conveying system. Material enters at the top of the valve and exits at the bottom of the valve. Essentially, material falls, under gravity, from the hopper into a sector, the sector rotates, and the material falls into the conveying system. Each sector within the star feeder valve acts like an airlock. The rate of material injection into the conveying system is determined by the speed of rotation of the star valve.
In order to create xe2x80x9cairlockxe2x80x9d properties, the stars (or open sectors) must have extremely tight clearances to and between the internal valve body. Basically, the sectors rub against the valve body walls at all times. Because the sectors rub against the walls, material within the sectors experiences grinding or damage. Furthermore, abrasive materials, such as sand, alumna, and the like, will grind the sectors eventually reducing the airlock properties of the valve. Unfortunately, some materials are capable of packing or clumping when exposed to work as exerted by a rotating star valve. These materials often seize a star valve causing damage to the drive motor and valve internals.
Finally, even though the star valve has xe2x80x98airlockxe2x80x99 properties, it is not an efficient airlock. Each sector, as it rotates from the conveying side pressure to the atmospheric side pressure, must equalize in pressure. Thus, air is transferred from the conveying system to the atmosphere (in a pressure system) or from the atmosphere to the conveying system (in a vacuum system). In fact, many star valves incorporate a special venting system, which allows such transfer to occur externally to the valve.
Rotary plate valves may also be used to transfer material from a hopper to a process and are well known in the art. These valves do not grind the material, nor do they suffer the drawbacks of abrasion or binding found in the star feeder. The rotary plate valve consists of a rotating plate upon which material falls, under gravity, and a plow or scrapper. The plow moves across the plate and scraps material from the plate into a discharge port or opening within the valve. The depth of the plow and the speed of the rotating plate control the rate of transfer through the valve. However, the current art in rotary plate valves does not extend to pressure conveying systems. There are some rotary plate feeder valves that may be used under pressure, but as will be seen these are somewhat limited in their application. Specific examples of the prior art may be found in the following series of U.S. patents.
Bonnot, U.S. Pat. No. 1,679,398, discloses a Disk Feeder for use in the coal industry and is probably one of the earlier disk feeders. Coal (or a similar material) is fed from an offset hopper onto a rotating disk and a scrapper blade removes the coal from the disk. The offset hopper barely touches the rotating disk and has a hole cut on one side of the hopper that allows material to fall onto the disk. A rotary sleeve valve rotates about the hopper and adjusts the amount of material falling onto the hopper. The scrapper blade is fixed. This early apparatus was designed solely for use in an atmospheric pressure environment.
Scholz, U.S. Pat. No. 1,993,249, discloses a Fine Coal Feeder that is a variation of Bonnot. Scholz places a hopper over a rotating disk, and the eccentricity of the hopper may be varied from zero eccentricity (i.e., over the center of the disk) to maximum eccentricity (at the edge of the disk). The eccentricity adjustment provides adjustment of xe2x80x9cfeedxe2x80x9d to the system. Coal is then scrapped from the disk into a down-comer and into the place of use (in this case a boiler). Again, this device was designed for use at atmospheric pressure.
Wheldon, U.S. Pat. No. 2,213,508, discloses a Feeder for Pulverulent Material. Wheldon places a hopper over the center of a rotating disk. The hopper has an opening in the side next to the rotating disk, and an adjustable xe2x80x9cscrapperxe2x80x9d extends through the opening. The scrapper is hinged at one end and may swing into the hopper or line up against the wall of the hopper. In the later position, no material feeds from the system. As the scrapper is positioned into the hopper, material is caused to flow from the hopper, across the disk and onto a conveyor belt. Once more this device was designed for atmospheric pressure.
Shallock, U.S. Pat. No. 2,329,948, discloses a Feeder Means that is similar to Wheldon in that a hopper is placed over the center of the rotating disk. The means for controlling the removal of material is quite different and uses a triangular shaped extension inside the hopper that is in contact with the disk. The extension serves to hold a wedge valve that allows material to flow from the hopper onto the disk and serves also scrap the material from the disk. (Essentially the triangular extension serves two consecutive purposes. Material then follows the extension and drops from the disk into the process. This device was also designed for atmospheric pressure.
Weiste, U.S. Pat. No. 3,820,688, discloses a Material Dosaging Apparatus that is designed for use in pneumatic systems. The apparatus is designed to mix different amounts of material (up to four) into a common stream for conveying. Weiste uses a modified disk in the form of a tub with an outside wall and a center conical section with an opening. An ejector is placed immediately above the opening through which pneumatic conveying air is passed. The ejector causes a partial vacuum, which draws material from the rotating tub into the conveying system. Material falls onto the tub from circular supply hoppers through a form of gate valve. The gate valves regulate the quantity of material falling onto the rotating tub and consequently into the conveying system. The individual hoppers are vented to atmospheric pressure and atmospheric air is permitted to flow through the hoppers to assure movement of material from the hopper. It is apparent that the design requires the supply hoppers to be at atmospheric pressure and that material is xe2x80x9csuckedxe2x80x9d into the conveying system.
Nussbaum, U.S. Pat. No. 4,227,835, discloses an Apparatus for the Metered Supply of Powder to a Power Processing Unit that is designed to operate independently of atmospheric pressure. This device uses a rotating disk with a groove machined into the plate. A hopper, which is in tight direct contact with the moving disk, drops power into the groove on the disk. The powder moves around in the groove and is sucked from the groove by a suction device. The groove and the hopper may be operated up to 3.5 bars. Power is metered by two techniques. The first is a xe2x80x9cdoctorxe2x80x9d member located at the hopper that is designed to pass a specific fixed amount of powder onto (or into) the groove. The second uses rotation of the disk that sets the quantity of power that may be draw up by the suction device. This device, although capable of operation at pressure, depends on very close tolerances and would suffer considerable wear when used with abrasive materials. Furthermore, the unit would not work well with loose (corn chips) material.
Douche et al, U.S. Pat. Nos. 4,789,569 and 5,104,230 disclose a Process and Device for Metering Pulverulent Materials. This device is similar to the device of Nussbaum in that a disk with a metering groove is utilized. A xe2x80x9cdoctorxe2x80x9d is not used with the hopper that operates at atmospheric pressure. A series of pads extends from the hopper and wipe excess material from the plate so that only the groove is filled with material. A suction device then picks up the metered material. The device suffers from similar drawbacks, as does the Nussbaum device.
The Nussbaum and Douche devices rely on vacuum pickup of the metered material. In normal pneumatic operations, both sides of the conveying system can operate at pressures greater than atmospheric pressure. None of the devices in the prior art disclose a metering system that will operate at pressure in a pneumatic conveying system.
Thus, there remains a need in the pneumatic conveying industry for a feeder valve that will act as an airlock valve with a variable injection rate while not grinding or sheering the material that is being conveyed or injected into the conveying system. Additionally, there remains the need for a feeder valve that will reduce or even eliminate the possibility of material buildup and binding within the valve internals.
It is an overall object of the present invention to provide a feeder valve that does not suffer the disadvantages of the rotary star or groove feeder valve. The instant feeder valve is essentially a rotary plate feeder valve, with a perimeter seal extending about the circumference of the rotating plate except at the discharge opening (or discharge openings in other embodiments). The valve may incorporate a movable sleeve that controls the depth of the material being deposited on the rotary plate and incorporates a movable plow (or plows in other embodiments) that controls the material being deposited into the discharge opening. The rotating disk is driven by a speed controlled motor external to the valve. The shaft connecting the motor and the rotating plate is sealed. In normal use, both the valve internals and the storage hopper (bin, silo, or the like) is maintained at the conveying system pressure by pressure means attached to the storage facility that follows the conveying system pressure.
It is possible to keep the hopper at atmospheric pressure and the discharge port at a much higher pressure. This would require a backup shutdown valve that would prevent backflow, if the hopper ran out of material. The backup shutdown valve would be placed on the outlet side of the discharge port(s).
Additional discharge ports may be added to the valve (during manufacture) thus allowing one valve to feed two conveying systems, provided the systems operate at the same pressure. Additional plows and associated ports may be incorporated in the valve which will reject material that is too large or too small; thus, allowing the valve to discriminate between different material sizes. The valve is manufactured so that different flanges may be mated to the valve body; thus satisfying many sizes of pipe and flange with a given minimum number of valve bodies.